1. Field of the Invention
The present invention relates to a DC-DC converter to convert direct current voltage, particularly to a DC-DC converter which can perform step-up and step-down without switching step-up and step-down modes. For example, the present invention relates to a technique advantageous in applying to a DC-DC converter for supplying direct current power voltage to a charging apparatus which charges a secondary battery using a primary battery.
2. Description of Related Art
A secondary battery is used in portable electronic equipment such as a cell phone as a main power supply or auxiliary power supply. To charge the secondary battery of such portable electronic equipment, an AC-DC converter, called as an AC adaptor, to convert alternate current voltage into direct current voltage is generally used. Such AC-DC converter has a problem that the use thereof is limited to a certain location such as indoor. However, portable electronic equipment may run out of battery during outdoor use where there is no AC power supply. Thus, there has been a requirement for an apparatus which can charge a secondary battery at a location without an AC power supply.
The present inventors have hit on an idea of charging a secondary battery by use of a primary battery as a method to charge a secondary battery at a location without an AC power supply, and have considered the idea. There is given a problem of such method that a voltage of a primary battery decreases as charge proceeds. The inventors have considered that usage of a step-up DC-DC converter is effective in order to enable the charge of a secondary battery even when the charge has proceeded and a voltage of a primary battery has decreased.
On the other hand, according to a process to charge a secondary batter by use of a primary battery, it becomes necessary to charge a secondary battery when the secondary battery has discharged. Therefore, a voltage of a secondary battery is often lower than that of a primary battery in an early stage of the charge process, and thereafter, when the charge has proceeded to some extent, a voltage of the secondary battery becomes higher than that of the primary battery because an output voltage of the primary battery decreases and a voltage of the secondary battery has increases. Furthermore, it is required that current restriction is provided with a charging circuit so that a current value does not excess a certain value.
However, a normal step-up DC-DC converter as shown in FIG. 4 has a diode disposed in a forward direction with respect to an output terminal. Therefore, when a voltage of a secondary battery is lower than that of a primary battery, an output voltage is clamped to be a voltage lower by a voltage forwardly across the diode than an input voltage, as shown in FIG. 5A. As a result, it has been found that it is impossible to accomplish a function of controlling an output voltage and restricting an output current in a voltage range lower than a voltage of the primary battery only by applying the step-up DC-DC converter.
The present inventors considered that a DC-DC converter which once steps-up an output voltage of a primary battery and subsequently steps down it, i.e. a DC-DC converter comprising a step-up regulator at the former stage and a step-down regulator at the latter stage, is preferable for a DC-DC converter which is also effective in such situation.
A DC-DC converter of FIG. 2 is one using switching regulators for both the step-up regulators at the former stage and the step-down regulator at the latter stage. Although this DC-DC converter has an advantage of superior electric efficiency, it is problematic that there is difficulty in downsizing and producing at lower cost because regulators in the former and latter stages requires coils L1 and L2 and smoothing capacitors C1 and C2 respectively, which results a large number of parts.
A DC-DC converter of FIG. 3 is one using a switching regulator in a step-up circuit at the former stage and a series regulator in a step-down circuit at the latter stage. Although this DC-DC converter has an advantage of facility in reducing the number of parts and downsizing compared with the DC-DC converter of FIG. 2, it is still problematic that there is difficulty in efficiently downsizing and producing at lower cost because two control circuits are required.
A DC-DC converter disclosed in, for example, Japanese patent application publication laid-open No. 05-056634 can be given as an invention having a structure similar to a basic structure (relationship among a coil, diode and switching element) of a circuit of a DC-DC converter according to the present invention.
However, this earlier development is to combine a switching power supply with a charge pump so as to obtain an original stepped-up voltage and the double voltage thereof. This converter cannot output current at an output voltage continuously from lower to higher than an input voltage, unlike the present invention.